So, it becomes the biggest creature and draws you cards? Nice!
Simic’s other Mythic creature is on par with Master Biomancer and some would argue better. It just comes down as your biggest creature (barring instant speed removal on your huge dude) and draws you card equal to its power. Obviously, it’s pretty bad if it’s your post- Supreme Verdict top deck. It does allow you to keep the swarm going and draws you a ton of cards that you might need in order to rebuild if things go bad. You also might want to open one up in your Release/Pre-Release if you plan on winning.
The creature I would most compare this to is Primeval Titan. Both provide you with great card advantage when they enter the battlefield. Titan continues the love in the form of more lands and if it survives long enough will win you the game. Zegana can just straight up give you 4-7 cards and will probably help you win the game if she manages to stay in play. Both cost 6 mana and both are well worth it. The legendary status hurts it in anything but Commander.
If you plan on playing casual, you can get in finite value out of her with something like Karakas to bounce her and re-cast over and over. While Karakas is not legal in Commander/EDH, I am sure your kitchen table group will not mind it when you play multiplayer casual games.
She also forms a lovely couple with Master Biomancer, for obvious reasons (with Biomancer in play she gets bigger and draws you more cards when she enters the battlefield). I would definitely add a Zegana to my Simic deck for just that reason.
Such a cute couple, aren’t they?
I once drew half my deck in one turn with that combo.